Aldehydes and ketones are important and extremely versatile intermediates in organic synthesis. Both compound classes exhibit a high reactivity of the C,O double bond, which enables numerous carbonyl reactions. The significance in modern organic synthesis is restricted only by limitations in the availability of these compound classes. Standard processes for preparing aldehydes and ketones are oxidations of corresponding alcohols, for which numerous methods such as catalytic vapor phase dehydrogenation or direct oxidation with molecular oxygen find use. It is also possible to use reagents such as hypohalic acids, heavy metal compounds such as silver carbonate, lead oxide, lead acetate, chromium oxides, ruthenates or else dimethyl sulfoxide, for example.
In modern organic synthesis, the significance of chemo-, regio- and stereoselective reagents is increasing explosively. When, for example, the intention is to convert a specific alcohol functionality to an aldehyde in a complex molecule with numerous functional groups, numerous methods from those mentioned, for example catalytic vapor phase dehydrogenation and direct oxidation with molecular oxygen, can no longer be used for selectivity reasons. The use of hypohalic acids is also restricted, since undesired side reactions such as overoxidation, halogenations or esterifications likewise occur, accompanied by low yields in some cases. The oxidation of primary alcohols to aldehydes or secondary alcohols to ketones with heavy metal compounds is always associated with the toxicity of the oxidizing agents in addition to the occurrence of by-products and the overoxidation.
There has to date been a lack of a highly selective solution to the problem of converting primary and secondary alcohols to the corresponding aldehydes and ketones, which can also be employed in complex multi-functional molecules. Although the known reagents can accomplish the desired transformations, other moieties are often likewise influenced. In many cases, the drastic conditions required epimerize even far-removed stereocenters. Moreover, the method to be developed should be heavy metal-free. In addition, the transformation should be employable under very mild conditions and the removal of the conversion products of the reagent used should be very simple.
It would therefore be very desirable to have a process which can convert primary and secondary alcohols by oxidation to the corresponding aldehydes and ketones but at the same time has very mild reaction conditions and a simplified workup, and is additionally usable in economically utilizable processes. The known reagents do not solve this problem, as will be demonstrated using some examples: although DMSO in combination with acetic anhydride can accomplish the reactions mentioned, this process only has restricted possible uses, since low yields are obtained in most cases. By-products are often formed in significant amounts, in particular via a Pummerer rearrangement. The oxidations of primary alcohols to aldehydes with DMSO in combination with trifluoroacetic anhydride can lead to explosions and must therefore be carried out at low temperatures at which, though, many complex molecules and natural substances are often no longer sufficiently soluble. The oxidation of primary alcohols with DMSO and thionyl chloride or oxalyl chloride must likewise be accomplished at low temperatures. However, these reagents can no longer be used when the molecules to be oxidized contain functional groups which can react with thionyl chloride or oxalyl chloride. It is likewise possible to carry out the desired transformation to aldehydes with DCC. However, the dicyclohexylurea formed as a conversion product can often barely be removed from the product or only by increased purification complexity. The use of water-soluble DCC derivatives is usually characterized by their very high cost, the instability of the intermediates in the oxidation and reduced effectiveness of the oxidizing agent.